As a conventional technology, Patent Document 1 proposes the Fabry-Perot interferometer providing the high-reflectance mirror using the optical multilayer. The Fabry-Perot interferometer includes lower and upper mirrors that are arranged opposite to each other with an air gap between them. According to Patent Document 1, the lower mirror is formed by alternately layering a silicon dioxide layer and a Poly-Si layer on a substrate. The upper mirror is formed by alternately layering a silicon layer and a dioxide layer. The optical multilayer is used to configure the high-reflectance mirror.                Patent Document 1: JP-3457373        
The optical multilayer, however, largely depends on the wavelength and narrows the mirror's high-reflectance band. The Fabry-Perot interferometer's spectroscopy band corresponds to the mirror's high-reflectance band. The Fabry-Perot interferometer using the mirror with a narrow high-reflectance band thereby narrows the spectroscopy band.